Resistance elements and method of making same



Oct. 20, 1959 J. c. PERRY, JR 2,90 ,7

RESISTANCE ELEMENTS AND METHOD OF MAKING SAME Filed Aug. 30. 1955 Fla].FIG. 2. 1716.5. 1 16.6.

iiiiiiimlili lllrii INVENTOR. JOHN CHARLES PaaQnd/a BY HIS ATTORNEYS.HARE/s, K/ECH, FosnzR & Heme/s llnited States Patent 2,909,753RESISTANCE ELEMENTS AND METHOD OF MAKING SAME ilolni Charles Perry, In,San Gabriel, Califi, assignor, by

mesne assignments, to Beckman Instruments, Inc., F ulle'rton, Califl, acorporation of California Application August 30, 1955, Serial No.531,368

9 Claims. (Cl. 338-298) This invention relates to resistance elementsand to a method of making same. More particularly, the invention relatesto resistance elements suitable for use in potentiometer-s or the like.While applicable both to resistors of the deposited-film type and of thewire-wound type, the latter will be herein illustrated by way ofexample.

In the potentiometer industry two types of Wound resistor elementsarecommon, one having a round cross section and the other havingessentially a rectangular cross section. The latter are wound on flat,straight mandrels or cards made of insulating material, the resistancewire being wound about the card while the latter is being turned in alathe or winding machine. Later, the cards are bent around and clampedto or within a circular member of the potentiometer so as to dispose thecard in a circle with its edge exposed for contact by a rotating wiperarm. One reason for utilizing cards instead of round mandrels is toprovide added lengths of resistance wire in the finished potentiometerwithin the same radial space factor, permitting the design ofpotentiometers of added resistance value. The speed at which aresistance wire can be wound around a card is strictly limited becauseof the flapping action or unequal winding rate of the resistance wire asthe flat mandrel or card is turned.

It is an object of the present invention to windthe wire initially on around mandrel, thus permitting winding with much higher speed andaccuracy, and then to -deform or flatten the resulting wound structureto change fitS cross-sectional shape, the core and its winding beingthus simultaneously changed in shape. It is another object of theinvention to produce resistance elements in a t'less expensive way whilestill providing a greater accuracy in the finished product.

In the preferred practice of the invention, a tubular core of deformablematerial, usually a soft metal such as copper, is surrounded by aresistance member separated therefrom by suitable insulating material,opposed sides of the resulting tubular structure being then pressedtoward each other to produce an elongated cross section. It is an objectof the invention to produce resistance elements in this way.

While the resistance member may be'a deposited layer of resistancematerial applied to or surrounding the hollow core, it is usually ahelically wound resistance wire. If the wire is coated with a sheath ofinsulating material, it can be wound directly upon a metal mandrel, theinsulating material being later removed in a longitudinal zone to betraversed by the rotary wiper or contact of the potentiometer.Alternatively or in addition to this manner of insulating the resistancemember from the core, I may initially encompass the tubular core with asheath of insulating material, as by wrapping the core -with apressure-sensitive tape formed of insulating material and applied eitherlongitudinally or helically. This ---assures a higher breakdown voltagerating between the core and the resistance member. It is an object ofthe invention to apply such a sheath of pressure-sensitive tape to atubular core before placing the resistance memher in position.

The use of a sheath of deformable insulating material immediately aroundthe tubular core is particularly desirable with wire-wound resistancesbecause the resistance wire can embed itself slightly in the sheathmaterial and thus more firmly hold its position during subsequentprocessing, including the deforming step. Irrespective of whether apressure-sensitive tape is used and irrespective of whether it isapplied helically or longitudinally to the core, it is an object of theinvention to apply a sheath of tape-like insulating material to the corebefore winding the bare or insulated resistance wire helicallytherearound whereby the winding operation will cause the wire to embeditself slightly in the sheath material.

A further object of the invention is to provide a re sistance elementand a method of making same in which the tubular core is initiallycylindrical and in which this core is sidewardly compressed after theresistance member has been applied, the sideward compression serving toflatten the structure either to a point where the opposed sides of thetubular core are in contact or to a point where such opposed sides arecloser together than initially to produce a hollow-core resistanceelement. In the latter instance, the unit will provide a flow pathwithin the core through which a cooling fluid or the like may 'becirculated if desired. The pressing or sideward compression will behereinafter referred to as flattening, irrespective of whether theopposed side walls are brought into engagement with each other, the termbeing inclusive of any operation wherein the opposed side walls arebrought closer to each other to elongate the cross section of the unit.

A further object of the invention is to provide a re.- sistance elementand a method of making same in which the flattened unit is subsequentlydeformed into circular or helical shape with either a flat side or anedge dis.- posed outwardly. It is an important advantage of theinvention that this operation shapes the unit to a permanent form. Thisis in contradistinction to prior attempts to deform units in which theresistance wire is wound around a flexible card and wherein the ends ofthe flexible card tend to spring back into straight form. Even if such aflexible unit is tightly clamped in place, it does not usually form atrue circle. On the other hand, the permanently deformable core of thepresent invention permits bending of the unit into a permanent circularor helical form. The result is a more accurate location of theresistance wire so that a potentiometer of greater linearity or accuracycan be produced.

It is an object of the invention to provide a continuous method ofmaking a wire-wound resistance element from a spool of resistance wireand a reel of tubular core material in which the tubular core materialis passed by the winding station in a continuous piece and theresistance wire is continuously wound on the core as it passes thewinding station, the wound unit being flattened and then being bent intoa coil prior to being cut into indi vidual resistance elements.

A further object of the invention is to provide a resistance element ofcontrolled resistance per unit of length and a novel method of makingsame in which a tubular core is varied in cross section or in peripheraldimension while still in tubular form, the resistance member being thenapplied and the unit then flattened in the manner referred to. above. Itis thus possible to produce resist ance elements of tapered or nonlinearresistance.

The invention alsovcomprises novel details of construction and novelcombinations and arrangements of parts which will more fully appear inthe course of the following description. The drawing merely shows andthe description merely describes preferred embodiments of the presentinvention which are given by way of illustration or example.

In the drawing:

Fig. 1 is a view of a tubular core in the process of being wound withtape and a resistance wire;

Fig. 2 is a cross-sectional view of the structure of Pi 1;

Fig. 3 is a view similar to Fig. 1 showing an alternative way ofsheathing the core with an insulating material before winding theresistance wire in place;

Fig. 4 is a cross-sectional view of the structure of Fig. 3;

Fig. 5 is a view of the wound unit of Fig. 1 after the flatteningoperation;

Fig. 6 is a cross-sectional View of the structure of Pi 5;

7 is a side view of a single-turn resistance element made according tothe teachings of the invention;

Fig. 8 is an end view of a resistance element of Fig. 7;

Fig. 9 is a side view of a multiple-turn flat-wound resistance elementformed according to the teachings of the invention;

Fig. 10 is a side view of a multiple-turn edge-wound resistance elementof the invention;

Fig. 11 is a side view of a resistance wire wound on a tubular corehaving a varying cross section;

Fig. 12 is a cross-sectional view of the structure of Fig. 11 takenalong the line 1212 thereof;

Fig. 13 is a side view of the wound core of Fig. 11 after the flatteningoperation; and

Fig. 14 is a cross-sectional view of the structure of Fig. 13 takenalong the line 14-14 thereof. 7

Referring particularly to Figs. 14, the preferred practice of theinvention starts with a tubular core 10 made of a malleable material,typically copper, aluminum, or the like, which may be subsequentlyflattened without rupture and without materially affecting itsmechanical properties. In a continuous operation, this tubular coremoves longitudinally past a winding station while in cylindrical form. Athin-walled copper tubing is a typical tubular core 10. i

If desired, the tubular core 10 may be coated with an insulatingmaterial prior to being wound with resistance wire. The preferred way ofaccomplishing this result is shown in Fig. 1 which suggests theprogressive winding of a tape 11 of insulating material helically aboutthe core 14 in edge-to-edge manner immediately ahead of the windingstation. Preferably this tape is a pressuresensitive adhesive tape madeof insulating material. This material should preferably be sufficientlysoft to permit the later-wound resistance wire to embed slightlytherein. Instead of applying the tape 11 helically, as suggested in Fig.1, a wider tape 11 may be applied longitudinally in encirclingrelationship with the core 10, the edges of the tape either abutting or,as shown in Fig. 3, overlapping slightly. In either instance, the tapecan be applied progressively as the core moves longitudinally relativeto the winding station. It should be clear, however, that it is notalways necessary to apply such a sheath of insulating material about thecore It) before it is wound.

At the winding station, a resistance wire 14 is wound helically aboutthe core 10. This wire is preferably coated with an insulating material,although a bare wire can be used if the tape 11 or 11' is provided andif the "wire is space-wound on the tubular core.

The wound unit of Fig. 1 or Fig. 3 is next flattened by being sidewardlydeformed or pressed to bring opposed side walls 15 and 16 closertogether. This may be done by pressing the wound unit between the flatjaws of a press or by continuously passing the unit between suitablerollers. The degree of deformation or flattening 'will depend upon thedesired cross-sectional shape of the resistance element and may varyfrom a slight deforma- 'tion of the core to an oval shape to the shapeof Fig. 6, in which the opposed side walls 15 and 16 are parallel butstill spaced to provide an internal channel 17 which may be used as aflow path for a coolant fluid, or even to a form in which the opposedside walls 15 and 16 are brought into face-to-face contact as shown insection in Figs. 10 and 14. The resulting flattened resistance elementcan be cut into lengths for use in straight form or for bending into thecircular form of Figs. 7 and 8.

Desirably, however, the flattened unit is continuously formed into theflat-wound helix of Fig. 9 or the edge wound helix of Fig. 10, beingused in such helical shapes for multiturn potentiometers or being cutinto sections which can be subsequently deformed into a single plane toform resistance elements of any desired arcuate length up to a fullcircle. The flattened unit can be wound into the helical forms of Figs.9 and 10 by use of suitable forming equipment or by being wound about amandrel. While the single-turn unit of Figs. 7 and 8 is shown as beingflat-wound, it should be understood that such single-turn units may alsobe edge-wound in which event they may represent segments of anedge-wound helix such as in Fig. 10. 5

An important advantage of this method of making resistance elements liesin the fact that high speed and accuracy can be achieved, along witheconomy, particularly in a continuous winding operation. Thus, a reelcontaining many feet of the tubular core 10 may be used, the tubingbeing continuously fed past a winding station where the resistance wire14 and near which, if desired, the tape 11 or 11' is applied. The woundunit may be flattened in sections between the jaws of a press or may becontinuously passed between suitable rollers. The wound and flattenedunit may then be cut into short sections, if straight resistanceelements are desired. -If circular resistance elements are desired, theflattened, wound unit may be continuously formed into a flat-wound oredgewound helix which can be used as such or from which one or moresingle-turn units may be cut.

The invention may also be used to produce resistance elements havingvarying or nonlinear resistance characteristics, as suggested in Figs.11-14 which illustrate a rather extreme situation where the resistanceper unit of length increases and decreases in a localized zone. Inproducing such resistance elements, the tubular core 10 is first upsetor swaged while still in tubular form to produce a cross section varyingin peripheral length at positions progressively along the length of thetubular core. In Fig. 11, a tubular core 30 has been internally expandedto produce a locally expanded zone 32 of larger diameter. A resistancewire 34 is then wound about the periphery of the expanded andnormal-size zones of the core, the entire unit being then flattened toany desired extent. Fig. 14 suggests an extreme flattening in whichopposite side walls 35 and 36 of the tubular core 30 are brought intocontact with each other. Any flattening deforms the zone 32 as well asthe remaining length of the core 30. The resistance per unit of lengthin the flattened zone 32 will be different, as compared with theremaining length of the wound unit, because of the greater length of theresistance wire 34 wound in this zone. By controlling the degree ofinitial peripheral deviation, resistance elements of any desirednonlinear function can be produced. Alternatively, resistance elementsof extremely accurate linearity can be produced by correlating thecross-sectional peripheral length of the core with the diameter of theresistance wire. It should be understood also that while Fig. 11 suggesta localized internal expansion'of the tubular core to obtain a localizedincrease in resistance per unit of length, compression of a localizedportion of the tubular core will produce a section'of decreasedperiphery and thus a section of decreased resistance per unit of length.

If an insulated resistance wire is used and the resistance element is tobe a part of a potentiometer having a movable contact arm or wiper, azone of the insulation is later removed in the area swept by thecontact. This can be done either before or after the unit is formed intothe circular or helical shapes of Figs. 7-10.

Although several exemplary embodiments of the invention have beendisclosed and discussed, it will be understood that other applicationsof the invention are possible and that the embodiments disclosed aresubject to various changes, modifications and substitutions withoutnecessarily departing from the spirit of the invention.

1 claim as my invention:

1. In a resistance element, the combination of: a flattened tubular coreof malleable metal; and a plurality of turns of resistance wireencircling said core.

2. In a resistance element, the combination of: a core comprising alength of flattened tubing of malleable metal with said length of tubingbent into a self-supported ring to form a ring-shaped core; and aplurality of turns of resistance wire encircling said ring-shaped coreand insulated therefrom.

3. In a resistance element, the combination of: a helical corecomprising a length of flattened tubing of malleable metal with saidlength of tubing bent into a self-supported flat-wound first helix; anda resistance wire wound about such helical core as a second helix havingsaid first helix as an axis.

4. In a resistance element, the combination of a helical core comprisinga length of flattened tubing of malleable metal with said length oftubing bent into a self supported edge-wound first helix; and aresistance wire wound about such helical core as a second helix havingsaid first helix as an axis.

5. In a resistance element, the combination of: a flattened tubular corehaving a cross-sectional shape much wider measured along a major axisthan along a transverse minor axis, the width of the flattened coremeasured in the direction of said major axis varying in differentportions spaced lengthwise of said core; and a plurality of turns ofresistance wire encircling said portions.

6. In a nonlinear wire-wound resistance element, the combination of: aflattened tubular core differing in peripheral dimension at differentportions spaced along the length of the core; and a plurality of turnsof resistance wire encircling and insulated from said portions.

7. In a resistance element, the combination of: a hollow metal corehaving a cross-sectional shape much wider measured along a major axisthan along a transverse minor axis, said core having a fluid passagetherethrough from end to end thereof; and a plurality of turns ofresistance wire encircling said core.

8. In a resistance element, the combination of: a tubular core having across-sectional shape much wider measured along a major axis than alonga transverse minor axis, with said tubular core flattened to bring theinner surface of opposing sides of said core into contact, the width ofthe flattened core measured in the direction of said major axis varyingin different portions spaced lengthwise of said core; and a layer ofresistance material carried on the outer surface of said portions.

9. In a resistance element, the combination of: a flattened tubular corehaving a cross-sectional shape much Wider measured along a major axisthan along a transverse minor axis, said core having a fluid passagetherethrough from end to end thereof, the width of the flattened coremeasured in the direction of said major axis varying in differentportions spaced lengthwise of said core; and a layer of resistancematerial carried on the outer surface of said portions.

References Cited in the file of this patent UNITED STATES PATENTS1,026,377 Barringer May 14, 1912 1,093,792 Madsen Apr. 21, 19141,755,314 Carter Apr. 22, 1930 1,767,716 Stoekle June 24, 1930 1,957,188Wiggins May 1, 1934 2,247,869 Beers July 1, 1941 2,466,227 Gilman et alApr. 5, 1949 2,542,806 Ford et al. Feb. 20, 1951 2,754,569 Kornei July17, 1956 FOREIGN PATENTS 1,491 Great Britain July 22, 1908 12,886Australia May 31, 1933 of 1933

1. IN A RESISTANCE ELEMENT, THE COMBINATION OF: A FLATTENED TUBULAR COREOF MALLEABLE METAL; A PLURALITY OF TURNS OF RESISTANCE WIRE ENCIRCLINGSAID CORE.